Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for steering a seismic vessel that tows marine equipment so that the marine equipment follows a pre-determined track.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration employed to determine the image of earth's subsurface. Marine reflection seismology is based on using a controlled source of energy that sends energy into the earth. By measuring the time it takes for reflections to return to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A traditional system 100 for generating seismic waves and recording their reflections off geological structures present in the subsurface is illustrated in FIG. 1A. A vessel 101 tows an array of seismic receivers 102 provided on streamers 103, which may be disposed to have a variable-depth, i.e., make an angle with ocean's surface 104. The streamers may be disposed to have other spatial arrangements, i.e., horizontal. Vessel 101 also tows a seismic source array 106 configured to generate a seismic wave 108. Seismic source array 106 may include plural individual source elements 107. The individual source elements may be distributed in various patterns, e.g., circular, linear, at various depths in the water.
Seismic wave 108 propagates downward toward the seafloor 120 and penetrates the seafloor until eventually a reflecting structure 122 (reflector) reflects the seismic wave. The reflected seismic wave 124 propagates upward until it is detected by receiver 102 on streamer 103. Based on the data collected by receivers 102, an image of the subsurface is generated.
For some planned seismic surveys, the source array and streamers should follow globally pre-determined tracks (or pre-plot or pre-plot track) so that the entire subsurface of interest is surveyed. The term “track” is understood herein to include the intended or desired horizontal direction of travel with respect to the earth. The term “course” is different from the term “track” as it includes the intended or desired horizontal direction of travel with respect to the water. For completeness, the term “heading” means a horizontal direction in which the vessel actually points or heads at any instant, the term “track over the ground” means the path over the ground actually followed by the vessel, and the term “track made good” is the single resultant direction from a point of departure to a point of arrival at any given time. The term “path over the ground” may also be understood to include actual previous positions of a traveling point (e.g., a source array) relative to Earth, as recorded for example by a GPS on the source array. All these concepts are illustrated in FIG. 1B.
There are different strategies to acquire the relevant seismic data related to the area of interest. For exploration surveys, the equipment (i.e., source array and/or streamer spread) is usually steered for coverage to achieve a given number of seismic traces per bin (a definition for a bin is provided, for example, in U.S. Patent Publication No. 2014/0029379, the entire content of which is included herein by reference). For multi-vessel, wide azimuth surveys, the vessels usually follow pre-determined tracks instead of steering for coverage. On circular or “coil” shooting surveys, the vessels usually follow pre-determined tracks instead of steering for coverage. This would also be the case for more complex patterns (sinusoidal or curved for example) or even for shooting during the “line change.” With OBS (Ocean Bottom Seismic), cables and/or nodes laid on the seafloor, the vessels usually follow pre-determined track as steering for coverage is meaningless. For “monitoring” surveys but also on baseline survey, it is the repeatability of the seismic equipment positions that is at stake and therefore, the equipment is steered for position in order to match a previous survey (base survey) or in order to have a baseline survey easy to match. It has been proven that 4-dimensional (4D) noise is correlated with mis-positioning of the source and/or streamers. The pre-determined tracks for seismic equipment, typically the acoustic sources, are usually achieved by a combination of a manual vessel steering system and sometimes a source steering system. Usually, the vessel is automatically steered by an auto-pilot system or PID controller 10 (Robtrack or Kongsberg Cjoy PID systems are the most commonly used in seismic acquisition) so as to be at a given cross-line distance from the seismic equipment's “given track.” For example, for a single source vessel, the given track would be the pre-determined track or pre-plot that the center of source should be following. In some cases, the pre-plot line could be just a straight line. Thus, the source vessel's auto-pilot is supposed to follow the given track with a certain cross-line distance. The seismic navigation system compares the vessel position with the “given track” and sends information to the autopilot so that the vessel follows the “given track at a certain cross-line distance.” At the same time, the navigator is following the deviation between the equipment positions, typically the sources mid-point, and the previous survey's equipment position, typically the sources mid-point track, but it could be any other point real or not. Based on that difference, the operator determines the cross-line distance at which the vessel should follow the given track. This cross-line distance is relayed to the autopilot by the seismic navigation system. The navigation system does not steer the vessel, but informs the autopilot on where the vessel should be and where it is currently relative to where it should be. The navigator is using the comparison of the current equipment position with the previous equipment position (from the base survey) for this purpose.
However, the manual determination of the best cross-line distance at which to steer the vessel is a real challenge for the navigator. There are several main issues in the way it is currently implemented in the main navigation system, and in the way a navigator can manage it. For example, there is a delay between the setting of a new cross-line distance and when the autopilot system reaches its target. Depending on the setting of the PID controller in the autopilot, the vessel may overshoot its target before coming back. The operator should be able to overlook those effects. However, there is an even longer delay in between the time when the vessel reaches its new target and when the position of the equipment is affected by the modification. A very skilled navigator is able to process all this data to determine a good cross-line distance due to a change of current, for example, taking into account the change to come due to its previous settings and overlooking any overshot of the vessel steering. Usually, manual interaction can induce oscillations of the equipment due to the delays in the corrections. Once the navigator is aware that the source/receiver spread is moving away from the desired track, it is too late to avoid significant mis-positioning. Subsequent corrections via alterations of the vessel's heading are likely to lead to over-corrections and an oscillatory path over the ground of vessel and source/receiver spread about the required track. Small deviations may not be reacted upon which might lead to too slow reactions when the ocean current condition changes. The steering performance will also depend on the navigator's skill level and level of alertness.
In U.S. Pat. No. 8,391,102 (the '102 patent herein), a method is described about how to automate both the steering of the vessel and the determination of the new track to be followed by a tracking point (having steering capability) based on the difference between the tracking point on the seismic spread and the pre-determined track and for the vessel steering using the difference between the new track and the measured track (of the tracking point in the spread). Various inputs can be considered by the controller or the navigator, e.g., environmental current and winds.
According to this method, as in most or all the methods for 4D vessel steering, the pre-determined position of a point in the spread is compared to its measured position to control the vessel steering. As the objective of the survey is to get a point in the spread to follow a pre-determined track, comparing its measured position to the intended one and computing a residual (difference) may seem to be the obvious way to do it. However, the inventors have discovered that this is not very efficient. The residual is very noisy and is not the important parameter. One of the issues encountered by the method of the '102 patent is that the vessel may not follow a straight line, but it may present long period oscillations around a straight line, depending on the parameters of the autopilot's PID controller. These oscillations will be reflected, after a given time delay, by the seismic spread and they will appear as residual in the current methods and will trigger correcting commands on the vessel steering. In the example described here, there should not be any correcting command send to the vessel steering, so it will create even more oscillations. This issue is encountered with all traditional systems that compare the current position of the spread with an intended track, whether it is manual or automated. Experience shows that the autopilot parameters need to be adapted to the weather conditions and to the way the vessel reacts, which is based on the speed, the size of the spreads and the deflectors. Thus, those skilled in the art would appreciate that, from one area to the other and from one spread to the other and depending on the weather conditions, the autopilot PID parameters may not be always optimally tuned. It is why a novel steering method that is stable in those cases is needed.
Another issue faced by the vessel steering control system, both with automated version and manual version, is the delay between the change of position of the vessel and the change of position induced in the spread. When the navigator or the controller orders the vessel to move cross-line by a certain distance, it is difficult for the system or the navigator to determine if the change in the residual between the intended track of the spread and its current position is due to the command sent to the vessel or due to a new change in external conditions, for example, current.
Because it is difficult for the current systems to distinguish the effects of wind and currents versus the effect of the steering, they need to add as input current and wind measurements. Those measurements may have errors and by adding those, the complexity of the system increases, which affects its robustness and accuracy.
European Patent EP 1735 641B discloses a way to improve the stability issue due to the reaction delay noted above. According to this document, instead of requesting the vessel to move to a given cross-line distance relative to the predefined track, which will change the position of the spread element with a delay or response time, it computes, using a complex force model, optimum tracks to be followed by the source and receivers. As long as the trajectory of the source and receivers stays within a “no change” corridor, no commands are sent to the vessel, which improves the system's stability. However, the force model is quite complex and requires good environmental data and good calibration.
In one embodiment of EP 1735641B, to overcome the stability issues, the response times of some spread element are estimated and taken into account, particularly when drive commands are sent to the vessel. However, determining the response times of the system elements to issue targets to the vessel autopilot, which removes some possible errors, is a complex task that is undesirable for the system's operator.
The above-noted problems are exacerbated for four-dimensional (4D) geophysical imaging, which is becoming more desired today. For 4D geophysical imaging, accurately positioning the source array and/or the streamers is important. 4D geophysical imagining involves 3D seismic surveys repeated over a same subsurface at different moments in time to determine changes in subsurface geophysical structures. Thus, as the 3D survey is repeated in time, sometimes after a few months or years, it is desirable that sources being used to generate seismic waves be located as close as possible to the positions used in the previous survey over the subsurface. It has been proven that the best way to compare surveys is to have traces which are repeated as accurately as possible. The trace is based on the source position and the receiver position.
Thus, it is challenging with existing methods to position various source arrays and/or streamers, at different moments in time, at the same locations, given cross-currents, wind, waves, shallow water and navigation obstacles currently encountered by vessels performing seismic surveys.
Accordingly, it would be desirable to provide systems and methods that provide a simpler method for controlling a vessel's trajectory so that towed marine equipment, e.g., source array or streamers, is more accurately positioned at desired locations during seismic surveys.